1. Field of the Invention
This invention relates to wet etches, and more particularly to wet etches for shaping silicon carbide electronic microstructures, and polishing and shaping silicon carbide wafers and removing surface damage therefrom.
2. Description of the Prior Art
Silicon carbide (SiC) is a promising high power semiconductor material with microwave potential because of its high saturated electron velocity, high electric breakdown voltage and high maximum working temperature. The first two factors allow silicon carbide to be operated at high frequency and high electric energy density, respectively. The third factor allows operation in hot environments or higher levels of self-heating. Further, the rate in silicon carbide self heating is low because its high thermal conductivity makes removal of waste energy to thermal heat sinks particularly efficient.
Exploitation of silicon carbide semiconductors is now limited by the difficulty of growing large crystals, preparing high quality large diameter wafers and shaping the wafer surfaces into the final device shapes required.
This occurs because SiC has a mechanical hardness which approaches that of diamond and because there have been no damage free etches to remove the damage introduced by diamond cutting or reactive ion beam machining. One well known solution is high temperature (e.g., &gt;1050.degree. C.) oxidation followed by wet etch removal of the surface oxide that is formed. This process is difficult, expensive, limited in the depth that can be oxidized in a single step and can propogate damage back into previously undamaged SiC.
Wet etch is characterized by a chemical reaction between the solid semiconductor and chemicals dissolved in the liquid whereby the chemical bonds between atoms on the surface of the solid are altered in such a way that the altered surface atoms can themselves be dissolved and washed away. Etching can involve a single liquid or multiple liquids used sequentially, and additional agents to drive the reactions. Wet etches are low energy processes sensitive to surface chemistry and surface damage which do not create additional damage.
It is often desirable to identify the 6H c-axis faces (carbon and silicon) because each face yields different electrical and mechanical properties and different epitaxial growth behavior. Conventional methods of identification require oxidation at temperatures of 1150.degree. C. for several hours to detect a relatively small difference in thermal oxidation rate.
Silicon carbide is a very hard material and is very difficult to cut and polish. Conventional methods of silicon carbide wafer preparation utilize diamond saws to cut rough silicon carbide wafers followed by high pressure diamond grit polishing to achieve mirror finish and plane parallel wafers. Further, direct polishing of a silicon carbide wafer with diamond removes silicon carbide damaged by cutting while introducing its own polishing damage.
Finally, it is well known in the state of the art that achieving the high electric energy densities (high breakdown voltage) predicted for SiC depends on removing damaged SiC surface material since that material acts as a low voltage breakdown path.
Therefore, there is a need for a wet etch usable with silicon carbide to prepare, polish and shape large diameter silicon carbide wafers.